Imidazolinone compounds, for instance, those described in U.S. Pat. Nos. 4,188,487; 4,798,619 and 5,334,576, are highly potent, broad spectrum, environmentally benign, herbicidal agents. In general, the herbicidal activity of the R-isomer is approximately 1.8 times that of the racemic imidazolinone compound. Stereospecific processes to prepare chiral imidazolinone herbicidal agents, either directly or indirectly, from (R)2-amino-2,3-dimethylbutyronitrile are described in U.S. Pat. No. 4,683,324 and co-pending patent application Ser. No. 09/304,401, filed on May 3, 1999.
Said nitrile is prepared by a two-step resolution of racemic 2-amino-2,3-dimethylbutyronitrile using D-(xe2x88x92)-tartaric acid as the resolving agent. D-tartatic acid does not occur in abundance in nature, and methods for its production are limited. Although D-tartaric acid is commmercially available, it is expensive and is only available in limited quantities.
Therefore, it is an object of the present invention to provide a process for the recovery of D-tartaric acid from the two-step resolution of racemic 2-amino-2,3-dimethylbutyronitrile.
It is another object of this invention to provide a process for the recycle of said recovered D-tartaric acid in the resolution process.
It is a feature of this invention that the processes provided thereby may be used for repeated recovery and reuse of D-tartaric acid in said two-step resolution.
The present invention provides a process for the recovery of essentially enantiomerically pure D-tartaric acid from a waste stream containing D-tartrate salts which comprises acidifying said waste stream to a pH of about 2.5 to 4.5 to obtain a crystalline alkali metal hydrogen D-tartrate; and reacting said alkali metal hydrogen D-tartrate with an acid, optionally in the presence of a solvent.
The present invention also provides a process for the recycle of recovered D-tartaric acid in the continuous resolution of racemic 2-amino-2,3-dimethyl-butyronitrile.
Imidazolinone compounds such as those described in U.S. Pat. Nos. 4,188,487, 4,798,619 and 5,334,576 are highly potent, broad spectrum, environmentally benign, herbicidal agents. Chiral imidazolinone compounds having the (R) configuration demonstrate an increase in herbicidal activity over the corresponding racemic mixture. The preparation of said chiral compounds by the resolution of racemic 2-amino-2,3-dimethylbutyronitrile, hydrolysis of the resultant (R) 2-amino-2,3-dimethyl-butyronitrile to the corresponding (R)2-amino-2,3-dimethylbutyramide intermediate, and subsequent elaboration of this intermediate to the (R)imidazolinone herbicidal product is described in U.S. Pat. No. 4,683,324. The preparation of chiral imidazolinone compounds having substantially complete retention of enantiomeric purity directly from the (R)aminonitrile starting material to the final chiral imidazolinone herbicidal product is described in co-pending patent application Ser. No. 09/304,401, filed on May 3, 1999.
In general, the two-step resolution described comprises a first resolution step in which racemic 2-amino-2,3-dimethylbutyronitrile (II) in C1-C4alkanol is treated with D-tartaric acid (I) to afford the D-tartrate salt of (R)2-amino-2,4-dimethylbutyronitrile (III), which crystallizes from solution. Because said aminonitrile partially decomposes in the process of this kinetic resolution, the methanol mother liquor contains varying amounts of ammonium D-tartrate (IV), which is ordinarily discarded. This first resolution step is shown in Flow Diagram I wherein the C1-C4alkanol is methanol. 
In the second resolution step, (R)2-amino-2,3-dimethylbutyronitrile (VI) is liberated from its D-tartrate salt (III) by treatment with an alkali metal hydroxide in the presence of a minimum amount of water and a water-immiscible solvent, such as toluene. This second resolution step yields an aqueous phase containing a full equivalent of the di(alkali metal) salt of D-tartaric acid (V), which is also ordinarily discarded. This second resolution step is illustrated in flow diagram II wherein M is an alkali metal and the water immiscible solvent is toluene. 
Thus both resolution steps give rise to waste streams containing D-tartrate salts.
Although L-tartaric acid is natural tartaric acid which occurs widely in nature, either as the free acid or in combination with potassium, calcium or magnesium, D-tartaric acid does not occur widely in nature and is commercially available only in limited quantities. Further, existing methods for producing D-tartaric acid are limited. Surprisingly, it has now been found that D-tartaric acid may be recovered in high yield and in essentially enantiomerically pure form from the waste streams produced in the resolution of racemic 2-amino-2,3-dimethylbutyronitrile. Advantageously, the recovered D-tartaric acid may be recycled for use in the same resolution of said aminonitrile. Beneficially, the processes of this invention may be run repetitively, i.e., D-tartaric acid may be repeatedly recovered and recycled in a continuous resolution of racemic 2-amino-2,3-dimethylbutyronitrile, allowing for a sustainable resolution process.
In accordance with the process of the invention the di(alkali metal) D-tartrate or ammonium D-tartrate waste streams produced in the resolution of the above-said aminonitrile are acidified to a pH of about 2.5 to 4.5, preferably 3.0 to 4.0, most preferably about 3.0. The acidification is preferably conducted with hydrochloric or sulfuric acid, to form the crystalline mono-basic hydrogen D-tartrate (VII) and said hydrogen D-tartrate is treated with at least one molar equivalent of an acid, optionally in the presence of a solvent, preferably an aliphatic alkanol, more preferably methanol or ethanol, to give essentially enantiomerically pure D-tartarc acid (I). The process of the invention is illustrated in flow diagram III wherein M is an alkali metal. 
The recovered D-tartaric acid (I) may then be utilized directly in the first resolution step by adding the recovered D-tartaric acid to a solution of racemic 2-amino-2,4-dimethylbutyronitrile in a water-immiscible solvent, such as toluene, to yield the corresponding D-tartrate salt (III) as shown hereinabove in flow diagram I.
It is also intended that the processes of this invention embrace the recovery and recycle of L-tartaric acid in a resolution of racemic 2-amino-2,3-dimethyl-butyronitrile to produce (S)2-amino-2,3-dimethylbutyro-nitrile, such as that described in U.S. Pat. No. 4,683,324.
Acids suitable for use in the process of the invention include mineral acids such as hydrogen halides, sulfuric acid, phosphoric acid, or the like, preferably hydrochloric acid or sulfuric acid.
Solvents suitable for use in the inventive process include polar solvents, preferably water miscible. Preferable solvents include aliphatic alkanols such as methanol, ethanol, propanol, isopropanol, or the like, preferably methanol or ethanol, more preferably ethanol.
Alkali metals include sodium, potassium, or lithium, preferably sodium or postassium.
In general, reaction temperatures for the inventive process are directly related to reaction rate, that is increased reaction temperature leads to increased reaction rate. However, excessively high reaction temperatures are to be avoided. Suitable reaction temperatures may be about 0xc2x0 C. to 50xc2x0 C., preferably about 5xc2x0 C. to 35xc2x0 C., more preferably about 10xc2x0 to 30xc2x0 C.
In actual practice, waste streams from a 2-step resolution of racemic 2-amino-2,3-dimethylbutyronitrile, combined or individually, are acidified to a pH of about 3 to form crystalline alkali metal hydrogen D-tartrate and said hydrogen D-tartrate is reacted with at least one molar equivalent of acid, preferably hydrochloric acid or sulfuric acid, optionally in the presence of a solvent, preferably an aliphatic alkanol, more preferably methanol or ethanol, to give the desired essentially enantiomerically pure D-tartartic acid. Advantageously, the crystalline alkali metal hydrogen D-tartrate may be isolated using conventional means such as filtration or, alternatively, may be carried on in the inventive process as is or as a concentrated slurry. Similarly, the recovered D-tartaric acid may be isolated using conventional techniques or recycled as is or as a concentrated slurry.
In order to facilitate a further understanding of the invention, the following examples are presented primarily for the purpose of illustrating certain more specific details thereof. The invention is not to be deemed limited thereby except as defined in the claims. HPLC designates high performance liquid chromatography. Unless otherwise indicated, all parts are parts by weight.